The need for effective radio frequency (RF) noise filtering and circuit protection from surge voltages is urgently required due to the rapid growth, use, and reliance of the community at-large on electronic devices.
Emitters and receivers of unwanted signals are an increasing problem. Such devices as computers, heart stimulators, hearing aids, cellular phones, pagers, radio transmitters, facsimile machines etc. are all becoming smaller and increasingly operate at higher frequencies. High voltage electrostatic discharge noise is produced by lightning and by new synthetic materials used in machines, clothing, and furnishings.
Low cost, effective, noise suppression is especially needed where noise interference is life threatening, such as in aircraft controls, air-bag release circuits and controlled vehicle braking circuits. A recent typical example of this hazardous situation, is a commercial radio station which has rendered the electronic braking systems in automobiles on an adjacent highway totally inoperative.
Thus far, effective noise suppressors have been expensive and have added circuit complexity. Recent developments have concentrated on improved cable connector filtering, and feedthrough devices due to the special urgency in that area.
Presently the most effective noise filters combine ferrite inductors with lumped capacitors to form "L" or ".pi." filter networks. Uncontrolled self-inductance and self-capacitance resides within these costly lumped filters and produces resonance points which enhance certain noise frequencies thus reducing their effectiveness.
Prior art RF filters and by-pass capacitors have strived for electrical insulation by high DC resistance, setting the required inter-electrode resistivity to over 1 M.OMEGA. per cm. This has severely restricted the choice of dielectric materials which can be used for noise suppression.
Metal Oxide Varistors such as the GE-MOV.RTM. originally pioneered by the General Electric Company, Auburn, N.Y., and presently manufactured by Harris Semiconductor, 1301 Woody Burke Road, Melbourne, Fla. 32902 have proved to be useful for voltage transient protection. Their non-linear conduction characteristic and rugged power absorption protects electronic circuits from noise voltage transients. Made from silicon carbide, selenium, or zinc oxide, these devices cannot be used directly with high frequency signal lines. Their inherent high capacitance, swamps, distorts, and severely attenuates high frequency signals. Unless they are indirectly connected to the lines using inductors, zener diodes, gas discharge tubes, and the like, these isolation circuits add complexity, high cost, and severely compromise the effectiveness of the varistor.
It is apparent that a more effective, low cost, RF noise filter, is required. A varistor-like transient noise suppression feature included in that filter, which preserved the integrity of high frequency signals would solve many troublesome noise problems.